Engine spark can be affected by humidity, among other factors. For example, various humidity levels, possibly depending on other factors such as an ambient temperature, can increase or decrease the propensity for knock, i.e., mistimed combustion of fuel, in an internal combustion engine. A vehicle engine controller or the like can adjust the timing of ignition of spark plug, pressure in a cylinder, etc., to improve engine spark conditions, i.e., reduce knock. However, sensors and the like used to determine humidity and other factors may be impose costs in terms of weight, size, money, etc., when used in a vehicle. Further, even if cost obstacles are overcome, in-vehicle sensors may not measure humidity and other factors with a desired accuracy for making adjustments to optimize engine spark.